Signal decoding with and without second synchronization word in a mobile communication system

ABSTRACT

A time division multiple access (TDMA) radio communication system ( 100 ) accommodates signal decoding by a mobile station ( 130 ) with and without a second synchronization word ( 214 ). The radio communication system includes at least one base station ( 102, 104 ) to transmit radio signals during a succession of time slots ( 202, 204, 206 ) including a current time slot ( 202 ) and a subsequent time slot ( 204 ). The radio signals include an indication of whether transmission of the subsequent time slot is guaranteed. The system further includes at least one mobile station to receive the radio signals in accordance with the indication.

RELATED APPLICATIONS

This non-provisional application claims priority of provisionalapplication Ser. No. 60/085,710, filed May 15, 1998, in the name of PaulW. Dent and Krister Raith.

BACKGROUND OF THE INVENTION

The present invention relates generally to transmitting and receivingsignals in a mobile radio communication system. In particular thepresent invention relates to decoding signals in a mobile radiocommunication system which provides for suspending transmission from abase station of the system during selected time periods.

In time division multiple access (TDMA) radio communication systems,each transmitter in the system is assigned or allocated a time slot fortransmitting radio signals. All other transmitters are quiet during thistime slot so that the intended receiver can clearly receive the radiosignals. If the radio communication system is a two-way mobile system,such as a cellular radio telephone system, a radio carrier frequency ina first frequency band is divided into time slots for communicating froma base station to a plurality of mobile stations, while a radio carrierfrequency in a second frequency band is divided into time slots forcommunicating from different ones of the plurality of mobile stations tothe base station. The base station communicates with mobile stations ina fixed geographic area near the base station. Other adjacent basestations serve mobile stations in adjacent areas.

During some of its time slots, the base station transmits control andtiming information. This information includes, for example, the identityof a mobile station for which a received call is intended, on a pagingchannel, and synchronization and timing information on a synchronizationor sync channel. A mobile station in the system receives thesynchronization channel and synchronizes its timing with base stationtiming using information in the synchronization channel. Usually, eachtime slot also includes a known pattern of data to facilitate the mobilestation estimating the channel phase and amplitude which is useful fordecoding the symbols transmitted in the slot.

It is known to use a synchronization word at the beginning of thecurrent time slot as well as the sync word at the beginning of the nextslot and at the end of the current slot to improve the reception anddecoding of symbols. Because of a fade or other interference on thechannel, the current or first synchronization word may not be reliablyreceived. By storing the received signal samples including the secondsynchronization word, the stored signal samples may be decoded usingeither the first or second synchronization word or both. This techniqueis described in U.S. Pat. No. 5,335,250, issued on Aug. 2, 1994, toDent, et al., and in U.S. Pat. No. 5,841,816, issued on Nov. 24, 1998,to Dent, et al. Both of these patents are commonly assigned with thepresent patent application and are incorporated herein by reference.Some mobile stations are presently in use which rely on reception of thesecond synchronization word for optimal performance.

In some mobile radio systems, it is proposed to suspend or altertransmission of information including synchronization words when a timeslot is unallocated. If the time slot is unallocated, no mobile stationis presently using that time slot. The could result in the phase,amplitude or direction of transmission by the base station changing inthe subsequent time slot with respect to the current time slot whichwould, without using the invention, degrade the operation of theexisting mobile stations. The transmission of information by the basestation in an unallocated time slot is however desired to be avoided toreduce interference for receivers in surrounding areas. Elimination ofthe transmission of the synchronization word and other informationduring unallocated time slots will reduce or eliminate this co-channelinterference improving overall performance in the radio system. Thisimprovement is threatened by the degradation due to removing the mobilestation's ability to use the second synchronization word.

For a mobile station which relies on reception of the secondsynchronization word for improved communication, elimination of thesynchronization word or a change in a characteristic such as timing,phase or direction of transmission in unallocated time slots will createa problem if the unallocated time slot follows the time slot allocatedto that mobile station. The mobile station may not be able tosuccessfully decode the channel or may spend undue time searching forthe second synchronization word. Accordingly, there is a need for methodand apparatus for accommodating the elimination of transmission of someslots by base stations in a mobile radio communication system withoutdegrading performance of remaining slots.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of a radio communication system;

FIG. 2 is a diagram illustrating communication format in the radiocommunication system of FIG. 1;

FIG. 3 is a flow diagram illustrating a method for operating a basestation in the radio communication system of FIG. 1;

FIG. 4 is a flow diagram illustrating a method for operating a mobilestation of a first type in the radio communication system of FIG. 1; and

FIG. 5 is a flow diagram illustrating a method for operating a mobilestation of a second type in the radio communication system of FIG. 1.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Referring now to the drawing, FIG. 1 shows a radio communication system100 including at least one base station 102, 104 and at least one mobilestation 130. The radio communication system 100 may be any two-way radiocommunication system, such as a cellular radiotelephone system, apersonal communication system (PCS), a trunked radio system, or other.In the illustrated embodiment, the radio communication system 100 is acellular radiotelephone system operated according to interim standardIS-136 as published by the Telecommunication IndustryAssociation/Electronic Industry Association (TIA/EIA).

In FIG. 1, the system 100 is illustrated with two base stations,including base station 102 and base station 104. In general, all basestations are configured similarly and base station structure andoperation will be described in conjunction with base station 102. Thebase stations of the system including base station 102 and base station104 are linked together to form a network. The network is controlled bya mobile switching center (MSC) 106. The MSC 106 is connected bylandline, telephone link or wireless link to each of the base stations102, 104. The MSC 106 controls overall network operation, providesconnection to the public switched telephone network (PSTN) and controlsinteroperation of the base stations to provide functions such ashandover of radio communication between two base stations and a singlemobile station in the system 100.

The base station 102 is exemplary of base station structure andoperation. The base station 102 includes a controller 110, a transmitter112, a receiver 114, a memory 116, a clock 118 and an antenna 120. Thecontroller 110 controls overall operation of the base station 102. Eachbase station provides two-way radio communication with one or mobilestations in a fixed geographic region near the base station. Thegeographic region may be divided into sectors, each sector served by anantenna or a portion of the antenna 120.

The transmitter 112 transmits radio signals using the antenna 120 tomobile stations such as mobile station 130 within a fixed geographicarea surrounding the base station 102. The transmitter 112 may includefunctions such as encoding, interleaving and modulation. The receiver114 converts radio signals received at the antenna 120 to digital datafor use by the base station 102. The receiver may include functions suchas filtering, demodulation and decoding. The memory 116 stores data andinstructions for operating the controller 110 and for use by othercircuitry within the mobile station 102.

The clock 118 provides a timebase for operating the base station 102. Ingeneral, equipment operating in the system 100 must be closelysynchronized including the base stations, the MSC 106 and mobilestations. The clock 118 receives synchronizing signals from the MSC 106and in turn is used to provide synchronizing signals to mobile stationssuch as mobile station 130.

It will be understood by those ordinarily skilled in the art that thebase stations 102, 104 may include other functions and other circuitryas well. These functions include managing call initiation with a mobilestation and hand over of communication with a mobile station from onebase station to another base station.

The mobile station 130 is exemplary of mobile stations which may operatein the radio communication system 100. The mobile station may be anyradio capable of two-way radio communication with a remote base station,such as a cellular telephone, PCS telephone or other two-way radio. Inthe illustrated embodiment, the mobile station 130 is a cellulartelephone operable according to IS-136. The mobile station 130 in theillustrated embodiment includes an antenna 132, a receive path 134, atransmit path 136, a controller 138, a memory 140, a user interface 142,a battery 144, a clock 146 and a synthesizer 148.

The receive path 134 receives radio signals detected at the antenna 132and produces digital data for use within the mobile station 130. Thereceive path 134 includes an analog front end 150, a demodulator 152 anda decoder 154. The analog front end 150 includes a low noise amplifierand suitable filters for detecting and filtering the radio signalsreceived at the antenna 132. The analog front end 150 may furtherinclude circuitry, such as a mixer coupled to the synthesizer 148, forshifting the frequency at which the radio signals are broadcast down toa lower frequency for more convenient processing in the mobile station130. The demodulator 152 processes received signal samples by using thesynchronization word or known symbol pattern to determine estimates ofthe multipath propagation channel and then uses the channel estimates toprocess signal samples to produce soft-decisions, compensated formultipath, and inter-symbol interference. The soft-decisions are thenfed from demodulator 152 to decoder 154 which performs error correctioncoding to produce error-corrected information to controller 138.

The transmit path 136 converts digital data produced by the controller138 to radio signals for transmission using the antenna 132. Thetransmission path 136 includes an encoder 160, a modulator 162 and atransmitter 164. The encoder 160 encodes the digital data provided bythe controller 138 into a format required for communication in thesystem 100 including interleaving. The encoded data is provided to themodulator 162 which uses the data to modulate a carrier signal providedby the synthesizer 148. The modulated carrier is provided to thetransmitter 164, which provides functions such as power amplificationand filtering. The amplified carrier is then applied to the antenna 132for transmission to a remote base station such as base stations 102,104.

The controller 138 controls overall functioning of the mobile station130. The controller 138 is preferably implemented as a microcontroller,a digital signal processor or a microprocessor or a combination ofthese, and operates in response to data and instructions stored in thememory 140. The user interface 142 allows control of the mobile stationby a user and typically includes a keypad, a microphone, a speaker and adisplay. The battery 144 provides operational power for the mobilestation 130. The clock 146 provides timing for circuitry of the mobilestation 130. In particular, the clock 146 provides timing signals foruse by the controller 138 so that the mobile station 130 including thecontroller 138 may remain synchronized with the rest of thecommunication system 100.

In the illustrated embodiment, the radio communication system 100 is atime division multiple access (TDMA) radio communication system. Thatis, in the system 100, each mobile transmitter in the system, whenactive, is assigned or allocated a time slot for transmitting radiosignals. All other mobile transmitters in the same cell using the samefrequency are quiet during this time slot so that the intended receivercan clearly receive the radio signals on that channel. Of course, inanother cell or on another frequency, other mobiles may independently beusing the same time slot.

One example of a TDMA radio communication system is a cellular telephonesystem according to interim standard IS-136. IS-136 defines acommunications frame having six time slots which however are normallyallocated in pairs to form a full-rate channel. The full-rate channel isthus really a three-slot TDMA system. By combining a slot on an uplinkfrequency with a slot on a downlink frequency, a duplex channel iscreated for two-way communication between the base station and aparticular mobile station.

FIG. 2 illustrates communication format in the radio communicationsystem of FIG. 1. FIG. 2 illustrates a portion of a communications frame200 including three time slots, a first time slot 202, a second timeslot 204 and a third time slot 206. The time slots 202, 204, 206 are alltransmitted by a base station in the system 100 of FIG. 1.

Each time slot includes a synchronization word. Thus, the first timeslot 202 includes a synchronization word 212, the second time slot 204includes a synchronization word 214 and the third time slot 206 includesa synchronization word 216. The synchronization word or sync wordcomprises a predetermined data pattern in a predetermined location nearthe beginning of each time slot. The synchronization word is used by areceiving mobile station for synchronizing timing and decoding thetransmitted time slot.

Each time slot and its associated synchronization word is intended bythe transmitting base station for reception by a single mobile stationin the vicinity of the base station. However, in the current IS-136system, all three time slots are always transmitted, so that theintended mobile station can receive other time slots such as subsequenttime slots as well as or even better than its intended time slot. InIS-136, the transmission format exhibits time-reversal symmetry, so thata mobile station may demodulate its information from the first sync wordforward or from the second sync word backward. However, in a futuresystem, a time slot may be omitted when there is no data to send,reducing interference to the unintended receiver, but depriving anintended receiver of the second synchronization word.

It is known to use the synchronization word of the current time slot andof a subsequent time slot to improve the reception and decoding symbolsby a mobile station. This technique is described in U.S. Pat. No.5,335,250, issued on Aug. 2, 1994, to Dent et al., and in U.S. Pat. No.5,841,816, issued on Nov. 24, 1998, to Dent et al. Both of these patentsare commonly assigned with the present patent application and areincorporated herein by reference. The decoding technique of theseincorporated references or other suitable decoding technique is referredto herein as a first decoding algorithm or technique. In accordance withthe presently disclosed embodiments, in situations where the basestation cannot guarantee transmission of a synchronization word in thesubsequent time slot, the mobile station will use a second decodingalgorithm or technique.

According to the decoding and demodulation techniques described in theincorporated references, a mobile station receives an assigned orallocated time slot of a TDMA frame period. Using a first decodingalgorithm, the mobile station decodes the received signals with the aidof known symbols called the first synchronization word that aretransmitted near the beginning of the allocated time slot and with theaid of known symbols transmitted near the beginning of a subsequent timeslot referred to herein as the second synchronization word. Statedalternatively, the base station transmits first predetermined datasymbols and unknown data symbols. The synchronization word or otherknown data pattern form the first predetermined data symbols. Thespecific data intended for the particular mobile station are unknowndata symbols. The unknown data symbols may be encoded speech data,control data or other data. In some systems, the base station transmitssecond predetermined data symbols, such as a second synchronization wordin the subsequent time slot.

In some systems, the known symbols at the beginning of the subsequenttime slot, for example, the second synchronization word, are notguaranteed to be transmitted by the base station. This may be done, forexample, to reduce co-channel interference for receivers in surroundingareas. If the subsequent time slot is not allocated to a mobile stationof the base station, the base station may suspend transmission duringthe unallocated time slot, including the synchronization word, to reduceco-channel interference. Alternatively, the base station may use adirectional antenna which permits broadcast of a time slot into oneparticular sector of the area served by the base station. This sectormay be different from the sector where the mobile station allocated tothe current time slot is located. Thus, the second time slot may not bereceived coherently with preceding slot data due to the change of beamdirection.

Because of these possibilities, the base station cannot guaranteetransmission of the second predetermined data symbols, such as thesynchronization word in the subsequent time slot. Further, thetransmission of the second predetermined data symbols, if it occurs, maynot be useful if the timing or phase or amplitude of the subsequent timeslot is substantially varied by the base station for any particularreason. Because the synchronization word is used by the mobile stationfor synchronizing its timing and reception to the base station, anychange in these parameters may cause a loss of synchronization andrequire re-synchronization of the mobile station to the base station.

When the known symbols, such as the synchronization word, at thebeginning of the subsequent time slot are not guaranteed to betransmitted by the base station, or are not guaranteed to be transmittedwith timing, phase, amplitude or direction continuity with respect tothe transmission in the current time slot, the base station according tothe invention provides a signal to the mobile station indicating thatthe second synchronization word cannot be relied upon. Statedalternatively, the base station transmits an indication whentransmission of the second predetermined data symbols is not guaranteed.

A mobile station decodes transmissions from the base station using atleast the first predetermined data symbols. If the signal transmitted bythe base station indicates that the transmission of the secondpredetermined data symbols or second synchronization word is reliable,the mobile station will use the first predetermined data symbols, thesecond predetermined data symbols or both to decode the time slotreceived from the base station according to the incorporated disclosureof a “first” algorithm. Any suitable algorithm described in theincorporated references or otherwise may be used.

When the mobile station receives the indication that the secondpredetermined data symbols or second synchronization word cannot berelied upon, the mobile station decodes signals received in a selectedtime slot using a second decoding algorithm. The second decodingalgorithm requires only known symbols transmitted in the selected timeslot. This second decoding algorithm may be any of the variationsdescribed in the above-incorporated references that are invoked bysetting the quality of the second synchronization word permanently tothe lowest quality value as long as the indication that the secondsynchronization word may not be relied upon is maintained. Any othersuitable decoding algorithm may also be used that makes use only of theone guaranteed synchronization word.

Thus the mobile station decodes the unknown data symbols using the firstpredetermined data symbols when the indication is transmitted, anddecodes the unknown data symbols using the first predetermined datasymbols and the second predetermined data symbols when no indication istransmitted. The indication may be any suitable data or signal containedwithin control signaling information transmitted by the base station tothe mobile station. In one embodiment, the indication is transmittedduring call setup or during handover. Call setup occurs when two-waycommunication is being initiated between the mobile station and the basestation. Call initiation occurs when the mobile station has indicatedthat a user desires to make an outgoing call or when the base stationhas signaled an incoming call to the mobile station on a paging channel.Handover occurs when a mobile station moves from the area of coverage ofa first base station to the area of coverage of a second base station.During both these occurrences, substantial control information isexchanged between the mobile station and the base station.

In one embodiment, the base station broadcasts an indication to allmobile stations in its covered region or in a sector of its coveredregion. This may be accomplished using the broadcast control channel(BCCH) known as the digital control channel or DCC. The indicationinforms mobile stations that the base station is using a mode ofoperation that does not guarantee the usability of the secondsynchronization word. In response, all mobile stations in the sector orcoverage area then use the second decoding algorithm.

In another embodiment, a mobile station provides an indicator to thebase station that the mobile station requires the second synchronizationword for optimum decoding of time slot information from the basestation. This is suitable for compatibility with mobile stations thathave this requirement. According to this embodiment, the mobile stationtransmits an indicator, the indicator indicating that the mobile stationrequires transmission of the second predetermined data symbols. Inresponse to the indicator, the base station according to the inventionreliably transmits the second predetermined data symbols even when nodata is transmitted in the following slot. The base station does notsuspend transmission of second synchronization word and does not varythe transmission phase or amplitude or any other necessarycharacteristic, but may omit transmission of data following the secondword, if no data need to be sent.

The indicator may comprise a predetermined data pattern or anoperational mode indicator for the mobile station. The indicator may betransmitted with control information provided by the mobile station tothe base station.

Alternatively, the indicator may be contained in standard informationtransmitted by the mobile station to the base station. For example, eachmobile station has a protocol version number which is transmitted to thebase station at the initiation of communication between the two. Aprotocol version number refers to the revision of the standard, such asIS-136, which the mobile station is using. In response to the protocolversion number or any other similar indicator, the base station candecide if the mobile station expects the second synchronization word orcan be reliably operated without transmission of the secondsynchronization word. Further, one of these two operational modes may beset as a default with the nondefault condition only being selected upona suitable determination by the base station. Still further, in yetanother embodiment, the base station may decide which mode to operate inby using an identifier such as the electronic serial number (ESN) of themobile station transmitted by the mobile station during call setup. Bycomparing the ESN for the mobile station with values stored in memory atthe base station or elsewhere on the communications network, the basestation can retrieve characteristics of that mobile station from asubscriber database, including which operational mode to provide for themobile station. Thus, the base station determines transmissionrequirements for the mobile station and transmits the predeterminedsecond data symbols in response to the transmission requirements for themobile station.

FIG. 3 is a flow diagram illustrating a method for operating a basestation in the radio communications system 100 of FIG. 1. The methodbegins at step 302. At step 304, the base station determines if themobile station (MS) requires a second synchronization word. Thisdetermination may be made according to any suitable method, such as byreceiving signaling information from the mobile station containing animplicit indicator that the mobile station requires the secondsynchronization word, or by receiving an identifier or other informationfrom the mobile station and comparing the identifier with stored data todetermine the mobile station's transmission requirements. In theembodiment illustrated in FIG. 1, the controller 110 of the base station102 forms a first means for determining a type of a particular mobilestation in radio communication with the base station. Further, thecontroller 110 forms a second means (110) for determining a type ofanother mobile station in radio communication with the base station. Thecontroller 110 may operate in conjunction with the memory 116 for thispurpose, or may operate in conjunction with other data processingequipment of the system, such as the MSC 106.

If the base station determines that the mobile station requires thesecond synchronization word, at step 306 the base station transmits afirst time slot including a first synchronization word. At step 308 thebase station then transmits at least the second synchronization word. Inthis manner, both the first predetermined data symbols (the firstsynchronization word) and the second predetermined data symbols (thesecond synchronization word) are provided to the mobile station foraccurate demodulation and decoding of the unknown data symbolstransmitted in the first time slot by the base station. In theembodiment of FIG. 1, the transmitter 112, alone or in conjunction withthe controller 110, forms a means responsive to the type of theparticular mobile station for transmitting radio signals to theparticular mobile station during a current time slot and during at leasta part of a subsequent time slot when the particular mobile station isof a first type. The method then ends at step 310.

If at step 304 the base station determined that the mobile station doesnot require the second synchronization word, at step 312 the basestation transmits the first time slot and the first synchronizationword. With regard to the embodiment of FIG. 1, the transmitter 112,alone or in conjunction with the controller 110, forms a meansresponsive to the type of the particular mobile station for transmittingradio signals to the particular mobile station during a current timeslot and radio signals adapted to a type of another mobile station whenthe particular mobile station is of a second type. Subsequently, at step314, the base station determines if the second time slot (i.e., the timeslot immediately following the first time slot) has been allocated. Thesecond time slot is allocated if the base station is in two-way radiocommunication with a second mobile station using the second time slot ofthe same radio channel. Transmission during the second or subsequenttime slot may be suspended or varied if the base station does notcommunicate with any mobile station during that time slot, or if themobile station allocated to that time slot is in a different direction.If the second time slot is allocated, at step 316 the base station willtransmit the second time slot and the second synchronization word, whichare required by the mobile station to which the second time slot isallocated.

If, at step 314, the base station determined that the second time slotis not allocated, at step 318, the base station determines if there is aneed to change its radio transmission. Such a need will exist, forexample, if the second time slot has been allocated to a mobile stationlocated in a different sector of the geographic area serviced by thebase station. If no change is required, control proceeds to step 316 andthe second time slot and second synchronization word are transmitted.However, if a change is required, at step 320, the base station makesthe necessary change in transmission and, at step 322, transmits thenext time slot. The method ends at step 310.

FIG. 4 is a flow diagram illustrating a method for operating a mobilestation of a first type in the radio communication system of FIG. 1. Thefirst type of mobile station requires or expects transmission of thesecond synchronization word by the base station. The method begins atstep 402.

At step 404, the mobile station remains in a loop, attempting to locatea control transmission from the base station (BS). If no suitabletransmission is located, the mobile station continues searching. Themobile station may be attempting to initiate a call by beginning two-wayradio communication with the base station, or may be attempting to handover communication from a first base station to a second base station tobe located.

In the context of FIG. 1, the receive path 134 forms a means forreceiving and decoding a signal from the remote base station. The analogfront end 150, the demodulator 152 and the decoder 154 are adapted toperform these functions. The design and implementation of thesecircuits, including both hardware and software, as well as alternativeembodiments, are described more fully in the incorporated references.The new indicator signal provided by the present invention indicateswhether the base station transmissions are of the first type or secondtype. The first type of transmission includes symbols such as asynchronization word in a subsequent time slot which the mobile station130 may use for decoding the base station transmission. The second typeof base station transmission does not reliably include the secondsynchronization word. The decoder 154, in combination with thecontroller 138, form a means responsive to the indication for decodingcommunications signals received from the base station 102, 104 in anallocated time slot of a TDMA frame period. The decoder 154 andcontroller 138 may be suitably implemented as any combination ofhardware or software for performing these functions.

In one embodiment, once a base station has been located, at step 406 themobile station determines to inform the base station of its need for asecond sync word for optimum performance. In other embodiments, nospecific implementation of step 406 may occur. Rather, the software orother routine which controls the operation of the mobile station maysimply perform the next step depending on the operational mode of themobile station.

If the mobile station does require a second synchronization word, it maytransmit an indicator at step 408. As described above, the indicator maybe data or other information contained in signaling, such as controlsignals. The control signals may be, for example, the electronic serialnumber (ESN) or mobile identification number (MIN) uniquely associatedwith the mobile station. The indicator indicates the type of mobilestation to the base station, which in turn determines if the mobilestation requires transmission of second predetermined data symbols in asubsequent time slot to its allocated time slot. Alternatively, theindicator can be a specific class mark or protocol version number whichis transmitted to the base station and is used by the base station todetermine that the mobile station requires transmission of the secondsynchronization word. Still further, the mobile station may merelytransmit identifying information, such as an identifier like itselectronic serial number, which is in turn used by the base station todetermine transmission requirements of the mobile station.

At step 408, the mobile station receives the first time slot transmittedby the base station and receives at least the second sync word. At step410 the mobile station demodulates the first time slot with the aid ofthe second sync word. More generally, the mobile station demodulates anddecodes the first time slot using any of the decoding techniquesdescribed above in the incorporated references or any other suitabletechnique. The mobile station determines at step 412 if more time slotsare forthcoming or if the call has been terminated. If the callcontinues, control returns to step 408. If the call has been terminated,the method then ends at step 416.

FIG. 5 is a flow diagram illustrating a method for operating a mobilestation of a second type in the radio communication system of FIG. 1.The second type of mobile station does not require or expecttransmission of the second synchronization word by the base station.However, in one embodiment, if the second sync word is available, themobile station of the second type uses the second sync word to optimizeits performance. The method begins at step 502. Step 504 proceeds likestep 404, described above in conjunction with the mobile station of thefirst type. At step 506, the mobile station transmits an indicator thatthe second sync word may be omitted.

If the mobile station does not require a second synchronization word, atstep 508 the mobile station of the second type may receive an indicationfrom the base station as to whether the second synchronization word isavailable. At step 510, the mobile station receives the first time slotincluding up to the second sync word. At step 512, the mobile stationchecks whether the second sync word has been indicated to be availableor useful. If so, the mobile station proceeds to step 514 and uses afirst demodulation algorithm using the first and/or the second syncword. If not, the mobile station proceeds to step 516 and uses a secondalgorithm to demodulate the first time slot using the first sync wordonly. Then at step 518, soft decisions from successive first slots,whether demodulated using the first algorithm at step 514 or the secondalgorithm at step 516, are de-interleaved and decoded.

If, at step 520, the call should continue, a return is made to step 510to receive the next first slot. Optionally, control returns to step 508if a per-slot indication of the availability of the second sync word isprovided. Otherwise, if the call has been terminated, the method ends atstep 522.

From the foregoing, it can be seen that the illustrated embodimentsprovide a method and apparatus for accommodating the elimination ormodification of transmission of some synchronization words by basestations in a mobile radio communication system. The base stationcommunicates an indication to mobile stations in the region served bythe base station that the transmission of subsequent synchronizationwords will be suspended or varied and are therefore not reliable. In analternative embodiment, a mobile station may transmit an indicator tothe base station that the mobile station requires the secondsynchronization word, in which case the base station will continue totransmit the second synchronization word for the use of the mobilestation.

These embodiments allow the deployment of improved base stations whichuse more advanced directive antenna arrays that can transmit differenttime slots of a TDMA frame period in different directions, adapted tothe position of the intended receiver. Alternatively, the embodimentspermit the use of adaptive power control where the transmission powerlevel in a time slot may be increased or decreased in dependence on thedistance of the intended receiver. Both of these variations affect thecharacteristics of the second synchronization word. Using theillustrated embodiments, such advanced base stations can be adapted torevert to transmission of the second synchronization word with signalcontinuity to the previous time slot in order to remain retrospectivelycompatible with mobile station receivers that rely upon the secondsynchronization word.

While a particular embodiment of the present invention has been shownand described, modifications may be made. For example, while thesynchronization word transmitted in each time slot has been shown foruse by the mobile station for demodulating and decoding transmissionsfrom a base station, any predetermined data or symbols contained in atime slot may be used by the mobile station. It is therefore intended inthe appended claims to cover all such changes and modifications whichfollow in the true spirit and scope of the invention.

1. A mobile station operable in a radio communication system (100)including one or more base stations transmitting during a succession oftime slots (202, 304, 206), the mobile station comprising: a decoderconfigured to identify in transmissions from the one or more basestations an indication whether a subsequent time slot can be relied onfor reception; and a demodulator for demodulating transmissions during acurrent time slot from the one or more base stations according to a oneof a first algorithm and a second algorithm in accordance with theindication.
 2. The mobile station of claim 1 wherein the decoder isconfigured to identify the indication in data received from the one ormore base stations.
 3. The mobile station of claim 2 wherein the decoderis configured to identify the indication in data received from the oneor more base stations during initiation of two way communication betweenthe mobile station and the one or more base stations.
 4. The mobilestation of claim 1 wherein the decoder is configured to identify theindication in the subsequent time slot.
 5. The mobile station of claim 1further comprising: a transmitter for transmitting signals to the remotebase station; and a circuit configured to format data for communicationto the remote base station, the data including an indicator whichindicates to the base station whether the mobile station acceptstransmissions of the first type.
 6. A base station configured foroperation in a radio communication system, the base station fortransmitting signals to one or more mobile stations in a predeterminedgeographic region, the mobile stations being of one of a first type anda second type, the base station comprising: first means for determininga type of a particular mobile station in radio communication with thebase station; second means for determining a type of another mobilestation in radio communication with the base station; and meansresponsive to the type of the particular mobile station for transmitting(a)radio signals to the particular mobile station during a current timeslot and during at least a part of a subsequent time slot when theparticular mobile station is of the first type, and (b)radio signals tothe particular mobile station during a current time slot and radiosignals adapted to the type of the other mobile station when theparticular mobile station is of a second type.
 7. The base station ofclaim 6 wherein the first means for determining comprises a circuitconfigured to determine the type of the particular mobile station basedon a transmission from the mobile station.
 8. The base station of claim6 wherein the means for transmitting adapts the radio signals accordingto one of changing a power level of transmission in the subsequent timeslot, changing a phase of transmission in the subsequent time slot,changing timing of transmission in a subsequent time slot, changingdirection of transmission in the subsequent time slot, changingmodulation of transmission, and changing burst format of transmission inthe subsequent time slot.
 9. A time division multiple access (TDMA)radio communication system comprising: at least one base station totransmit radio signals during a succession of time slots including acurrent time slot and a subsequent time slot, the radio signalsincluding an indication of whether transmission of the subsequent timeslot is guaranteed; and at least one mobile station to receive the radiosignals in accordance with the indication, wherein the at least onemobile station comprises a circuit to decode the radio signals usinginformation in the current time slot and information in the subsequenttime slot when the indication indicates that the transmission of thesubsequent time slot is guaranteed.
 10. A base station configured foroperation in a radio communication system, the base station fortransmitting signals to one or more mobile stations in a predeterminedgeographic region, the mobile stations being of one of a first type anda second type, the base station comprising: first means for determininga type of a particular mobile station in radio communication with thebase station; second means for determining a type of another mobilestation in radio communication with the base station; and meansresponsive to the type of the particular mobile station for transmitting(a) radio signals to the particular mobile station during a current timeslot and during at least part of a subsequent time slot when theparticular mobile station is of the first type, and (b) radio signals tothe particular mobile station during a current time slot and radiosignals adapted to the type of the other mobile station when theparticular mobile station is of a second type.